1. Field of the Invention
This invention relates to improvements in ultrasonic power supplies, and, more particularly, to improvements in ultrasonic power supplies of the type used for forming ball and stitch bonds in attaching wire leads between integrated circuit chips and carrier lead frames in the manufacture of integrated circuit devices.
2. Description of the Prior Art
At one stage during the fabrication of integrated circuit devices, wire leads are attached between contact points on an integrated circuit chip and a corresponding wire lead on a lead film or frame upon which the chip is carried. This is usually accomplished by a bonder which automatically locates a pad or point on the integrated circuit chip to which connection is to be made and brings a wire dispensing horn into contact with it. During the process, a small amount of wire material from which the lead is to be constructed is dispensed, and is melted by a torch, forming a ball. A measured amount of ultrasonic power is applied to the horn. This power causes the dispensed wire lead to bond to the integrated circuit, referred to in the art as a ball bond. Thereafter, an additional amount of wire is dispensed as the horn is moved to a location on the lead film upon which the circuit is carried. The lead is then bonded to the lead film, but the form of the bond is accomplished by transversly attaching the wire along its length to the foil lead with a so-called "stitch bond."
In the past, both ball bonds and stitch bonds were accomplished by applying to the wire dispensing horn a predetermined power pulse having voltage and current waveforms generally as shown in FIGS. 1 and 2. The envelope form of a typical voltage with respect to time at a typical frequency of, for instance, 62.5 kHz is shown in FIG. 1, and the envelope of the current produced in the horn during the bonding process with respect to time is shown in FIG. 2.
The bonding horns themselves generally include a piezoelectric device having an extremely high Q, and typical prior ultrasonic power supplies used with such bonding devices are programamble only in respect to time and peak to peak power. Thus, as shown, the voltage waveform applied is practically of a squared envelope pattern representing, in essence, merely the turning on and turning off the applied voltage. The particular frequency applied is specifically tuned to the extremely high Q piezoelectric horn device used. It can be seen that the current waveform is of a generally decaying nature, as opposed to the voltage waveform. This is because as the bond is being completed, the impedance of the bond function changes, thereby affecting the current waveform. The particular current waveform produced in uncontrolled, and, the waveform illustrated is merely one example of the numerous waveforms which might appear, depending upon many factors, as is known in the art.
Typical prior art bonders include separate means for controlling the voltage for different particular type bonds. For instance, some bonding apparatuses include a first program channel to set the parameters for effecting ball bonds and a second programmable channel for effecting stitch bonds. Thus, the amplitude and the time duration of each voltage pulse having the envelope waveform of FIG. 1 are individually adjusted depending upon the average parameters anticipated to be encountered in the bonding process. Thus, the applied power pulse typically can be adjusted manually, or, in some case, by preprogramming the amplitude desired, especially for a particular location or bond. This results in bonds of not optimum quality; in fact, sometimes, for instance, in the formation of stitch bonds, either an insufficient amount of power is applied, or, conversely, too much power is applied causing the bond to either be malformed or burned completely through. Additionally, the control of the power to the horn is uncontrolled once it is turned on until the time that it is turned off, resulting in the bond being as it fortuitously results.
In the attachment of the wire leads to the integrated circuit package, depending upon the orientation of the integrated circuit chip, which may not be the same from chip to chip, or from location to location, oftentimes prior apparatuses sense the contact between the lead carrying horn and the chip by pressure sensitive means, which, once touchdown is sensed, sometimes results in the horn having been pressed too hard into the substrate to form an effective bond.
It has been proposed to monitor the formation of wire bonds in real time in ultrasonic microelectronic wirebonding processes. For example, M. McBrearty et al, "REAL TIME NODESTRUCTIVE EVALUATION IN ULTRASONIC WIRE BONDING" published by Kulicke and Soffa Industries, Inc., Horsham, Pa. have proposed devleoping a signature of attempted wire bonds and assigning a grade to the signatures. Those bonds with a particular signature for low grade are then used to indicate either poor bonds or bonds attempted without wire.